The present invention relates in general to a subscriber's line circuit for a telecommunications network, particularly for a telephone network, and is of the type which includes at least one user's line loop connected to a user's terminal station, two sources of constant current arranged symmetrically to the terminal station in the user's line loop for keeping a constant current irrespective of the length of the line loop.
A line circuit of this kind is known for example from German published patent application No. 28 50 905. In this prior-art circuit, a source of constant current is applied both to the a-trunk line and to the b-trunk line of the user line circuit, in order to obtain symmetrical damping conditions of the voice-modulated alternating voltage, and thus to avoid crosstalk phenomena. Due to an inductive or capacitive coupling, relatively large hum voltages in the subscriber's line will occur, caused for example when overland lines or railroad lines extend parallel to the subscriber's line. As a consequence, the symmetrical arrangement of current sources for damping the voice-carrying voltages is no longer sufficient, especially when high standards for suppression of crosstalk are to be met, because the interference voltages cause an additional current to flow in unison with the frequency of the hum voltages, thus disturbing the symmetry of the alternating voice voltage. Since this known line circuit arrangement enables a simple monitoring of the operational conditions of the line loop, such as loop open, loop closed, ground key activated, it is desirable that the susceptibility of this monitoring function to interfering voltages be reduced, especially due to the fact that a disturbance signal is coupled into the line, the monitoring function can be impaired, for the damping of the crosstalk becomes ineffective.